In order to build faster and more complex integrated circuits, semiconductor device manufacturers have increased the number of components in the integrated circuits, while reducing the overall size of the circuit. The small circuit size requires multiple overlying metal interconnect layers to electrically connect the vast number of components within the integrated circuit. The multi-level metal interconnects are necessary in order to provide the large quantity of electrical connections necessary to electrically couple the density packed device components to each other, and to electrical circuitry within the device packaging.
With the advent of ultra-large-scale-integration (ULSI) semiconductor technology, multi-level interconnect layers must be fabricated at increasingly high-density levels. Each metal interconnect layer includes a large number of metal leads arrayed over and inter-level-dielectric (ILD) layer. The individual metal leads are fabricated using high resolution photolithographic and etching methods to have a very small line width. A high packing density of metal leads is obtained by placing the leads very close together, such that a very small line-space pitch is achieved.
In order to electrically couple the individual metal leads of a multi-level interconnect structure, vias openings are formed in the ILD layers and metal plugs are formed within the vias. The metal plugs provide an electrical conduit between overlying metal interconnect layers. Most often, the metal interconnect layers are composed of aluminum and aluminum alloys. Although aluminum and aluminum alloys can be used to form metallized vias plugs, in certain cases aluminum cannot be reliability deposited to form a high-reliability via plug. Accordingly, tungsten has become a widely used material for the formation of metallized via plugs. Tungsten possesses high electrical conductivity, and can be readily deposited into high aspect ratio vias. The aspect ratio of a via opening is determined by the ratio of the depth of the opening to the diameter of the opening. Typically, in a ULSI device, the aspect ratio of via openings can be 4:1 or higher.
Although tungsten can readily be used to form via plugs, the physical properties of tungsten differ substantially from the physical properties of aluminum and aluminum alloys. For example, tungsten is a mechanically hard, high-density metal having a high melting point. In contract, aluminum is a soft ductile metal having a relatively low melting point, and characterized by large grain structure. When electrons flow from hard metals, such as tungsten, to a softer metal, such as aluminum, the electron flow pushes atoms in the softer metal away from the interface. This phenomenon is known as electromigration and severely degrades the current handling capability of the metal interconnect structure. Because tungsten has a high density and melting point, it does not experience a mechanical deformation when subjected to a high electrical current. Moreover, tungsten does not self-diffuse when subjected to high electrical current. Therefore, electromigration of the softer metal in the direction of electron flow causes a void at the tungsten-aluminum interface.
To combat electromigration in aluminum interconnect structures, aluminum is alloyed with copper and other metals to increase its hardness and provide enhanced resistance to electromigration. Additionally, refractory metal barrier layers have been fabricated to reside between the tungsten plug and the metal interconnect layer. Use of refractory metal barrier layers is reasonably effective in reducing electromigration within the interconnect structure. However, even with the use of refractory metal barrier layers, severe degradation and device performance over long periods of time is still observed. For example, devices fabricated with aluminum alloy metallization and refractory metal barrier layers can suffer electromigration defects that reduce the useful lifetime below a commercially viable level. Accordingly, improvements are necessary in high-density, multi-level metal interconnect technology to enable the fabrication of devices with improved quality and enhanced lifetime performance.